Thin channel FET with recessed source/drains and extensions

ABSTRACT

A field effect transistor (FET), integrated circuit (IC) chip including the FETs and a method of forming the FETS. The devices have a thin channel, e.g., an ultra-thin (smaller than or equal to 10 nanometers (10 nm)) silicon on insulator (SOI) layer. Source/drain regions are located in recesses at either end of the thin channel and are substantially thicker (e.g., 30 nm) than the thin channel. Source/drain extensions and corresponding source/drain regions are self aligned to the FET gate and thin channel.

BACKGROUND OF INVENTION

The present invention is related to semiconductor devices andmanufacturing and more particularly to high performance field effecttransistors (FETS) and methods of manufacturing high performance FETS.

Typical semiconductor integrated circuit (IC) design goals include highperformance and density at minimum power. To minimize semiconductorcircuit power consumption, most ICs are made in the well-knowncomplementary insulated gate field effect transistor (FET) technologyknown as CMOS. A typical CMOS circuit drives a purely or nearly purecapacitive load and includes paired complementary devices, i.e., ann-type FET (NFET) paired with a corresponding p-type FET (PFET), usuallygated by the same signal. Performance depends upon how fast the CMOScircuit can charge and discharge the capacitive load, i.e., thecircuit's switching speed. Since the pair of devices have operatingcharacteristics that are, essentially, opposite each other, when onedevice (e.g., the NFET) is on and conducting (modeled simply as a closedswitch), the other device (the PFET) is off, not conducting (ideallymodeled as a simple open switch) and, vice versa. Thus, ideally, thereis no static or DC current path in a typical CMOS circuit and thecircuit load switches as fast as one switch can be closed and the otheropened.

A CMOS inverter, for example, is a PFET and NFET pair that are seriesconnected between a power supply voltage (V_(dd)) and ground (GND). Bothare gated by the same input and both drive the same a capacitive load.At one input signal state the PFET is on pulling the output high, PFETon current charging the load capacitance to V_(dd). At the oppositeinput signal state the NFET is on pulling the output low, NFET oncurrent discharging the load capacitance back to ground. Device oncurrent is related to gate, source and drain voltages and, dependingupon those voltages, the device may be modeled as a voltage controlledcurrent source or a resistor. Since series resistance, i.e., in thedevice drain or source, drops some voltage as current flows through thedevice, series resistances affect device voltages and so, affect(reduce) device current, slowing the charge or discharge of thecapacitive load. The switch is open, i.e., the device is off, when themagnitude of the gate to source voltage (V_(gs)) is less than somethreshold voltage (V_(T)) with respect to its source. So, ideally, anNFET is off below V_(T), and on, conducting current above V_(T).Similarly, a PFET is off when its gate is above its V_(T),i.e., lessnegative, and on below V_(T).

Semiconductor technology and chip manufacturing advances towards highercircuit switching frequency (circuit performance) and an increasednumber of transistors (circuit density) for more function from the samearea have resulted in a steadily decreasing chip feature size and,correspondingly, supply voltage. Generally, all other factors beingconstant, the active power consumed by a given unit increases linearlywith switching frequency. Thus, not withstanding the decrease of chipsupply voltage, chip power consumption has increased as well. Both atthe chip and system levels, cooling and packaging costs have escalatedas a natural result of this increase in chip power.

Unfortunately, as FET features have shrunk, device leakages includinggate leakages (i.e., gate to channel, gate to source or drain and gateinduced drain leakage (GIDL)) and source/drain junction leakages havebecome pronounced. Collectively, these leakages are included in what isknown as sort channel effects. In well known bulk technologies, forexample, short channel effects occur, in part, because dopant implantedin the device source/drain regions out-diffuses radially, such thatsource/drain regions extended below the device channels. This resultedin a buried leakage path between the source/drain regions and, in somecases in sub-surface channel shorts. Other leakage sources arise, forexample, as the distance is reduced between the source and drainjunction barrier layers, i.e., from reverse biased junctions. Typically,sub-threshold effects include what is known as subthreshold leakagecurrent, i.e., current flowing drain to source (I_(ds)) at gate biasesbelow threshold for NFETs and above for PFETs. Further, for a particulardevice, subthreshold current increases with the magnitude of thedevice's drain to source voltage (V_(ds)) and inversely with themagnitude of the device's V_(T), drain induced barrier lowering. Inaddition to the leakage, short channel effects also include what isknown as V_(T) roll-off, where the short channel device'scurrent-voltage (I-V) curve exhibits degraded definition.

Lightly doped drains (LDD) are one approach to reducing short channelproblems. Essentially, spacers are formed alongside FET gates atsource/drain regions. The spacer blocks or attenuates implanting dopantat the gates, spacing the source/drain diffusions away from the gate. Alightly doped region, typically implanted prior to spacer formation, isformed between the heavier source/drain regions and the gate to completethe device. Unfortunately, these lightly doped regions add seriesresistance at the source and drain of each device, which reduces devicecurrents and degrades circuit performance. Furthermore, as devicechannel lengths have shrunk well below one micron (1 μm), subthresholdproblems have become more pronounced and lightly doped drains does notsolve those problems.

Short channel effects improve inversely with body thickness. So,sub-threshold leakage and other short channel effects have beencontrolled and reduced in silicon on insulator (SOI), by thinning thesurface silicon layer, i.e., the device layer. In what is commonlyreferred fully depleted (FD) SOI on an ultra-thin SOI wafer, the siliconlayer is less than 50 nm. Ultra-thin SOI is the leading candidate tocontinue scaling gate to deep sub 40 nm and beyond. Ultra-thin SOIdevices operate at lower effective voltage fields. As a result, thedevices can be doped for higher mobility, which in turn increases devicecurrent and improves performance. Also, ultra-thin SOI devices have asteeper subthreshold current swing with current falling off sharply asV_(gs) drops below V_(T). Unfortunately, however, because source/drainregions are made from the same ultra-thin SOI layer, devices have higherexternal resistance.

So, to reduce this ultra-thin SOI device external resistance, thesemiconductor surface layer is selectively thickened, e.g., usingselective epitaxial silicon growth, to produce raised source and drain(RSD) regions. The raised source/drain regions have a largercross-sectional area and so, lower resistance per unit area (sheetresistance) and so, are effective in overcoming the external resistanceproblem. Unfortunately, raised source/drains above the silicon layersurface places parallel surface areas at each side of the gate,requiring gaps at gate sidewalls (e.g., spacers) to prevent shorts and,simultaneously causing increased parasitic gate capacitance between thegate and the RSD regions. For example, 30 nanometer (30 nm) RSD regionsmay increase overlap capacitance for an ultra-thin (˜10 nm) FET with 10nm sidewall spacers as much as 25 50% (0.08 0.2 femtoFarads (fF) permicron of width) depending upon spacer material. Further, the sidewallspacers add to device area, preventing RSD regions from being placed atthe channel ends. Thus, RSD requires a trade-off between reducingexternal resistance and accepting increased parasitic capacitance.

Both U.S. Pat. No. 6,420,218 B1 to Yu, entitled “Ultra-Thin Body SOI MOSTransistors Having Recessed Source And Drain Regions” and U.S. Pat. No.6,437,404 B1 to Xiang et al., entitled “Semiconductor-on InsulatorTransistor with Recessed Source and Drain” teach recessed source/drainregions as an approach to avoiding or reducing parasitic capacitance.Unfortunately, both Xiang et al. tolerates resistive extensions thatconnect the device channel to the recessed source/drain regions and thatfunction similarly to LDD, adding series source/drain resistance. Yuteaches forming an ultra-thin channel on defined source/drain recessesand the FET gate between the source/drain recesses, two layer below.Since Yu aligns the gate to the source/drain recess, i.e., because Yu'sdevices are not self aligned, Yu produces FETs with a relatively wideprocess variation (i.e., channel length and source/drain overlap), whichresults in wider spread for circuit performance, i.e., deviation beyonda nominal design point by a significantly larger number ofcircuits/chips. Thus, previously, one was faced with accepting parasiticdevice capacitances, series channel resistances and/or looser designtolerances.

Thus, there is a need to reduce external resistance for ultra-thin SOIdevices and while minimizing device on resistance.

SUMMARY OF INVENTION

It is a purpose of the invention to improve ultra-thin SOI performance;

It is another purpose of the invention to reduce ultra-thin SOI deviceexternal resistance;

It is yet another purpose of the invention to reduce external resistancein ultra-thin SOI devices without increasing device parasiticcapacitance.

The present invention relates to a field effect transistor (FET),integrated circuit (IC) chip including the FETs and a method of formingthe FETS. The devices have a thin channel, e.g., an ultra-thin (smallerthan or equal to 10 nanometers (10 nm)) silicon on insulator (SOI)layer. Source/drain regions are located in recesses at either end of thethin channel and are substantially thicker (e.g., 30 nm) than the thinchannel. Source/drain extensions and corresponding source/drain regionsare self aligned to the FET gate and thin channel.

BRIEF DESCRIPTION OF DRAWINGS

The foregoing and other objects, aspects and advantages will be betterunderstood from the following detailed description of a preferredembodiment of the invention with reference to the drawings, in which:

FIGS. 1A-C, each show an example of a preferred embodiment field effecttransistor (FET) according to the present invention;

FIG. 2 shows a flow diagram example of steps for forming FETs with selfaligned recessed extension and source/drain (ESD) areas on an ultra-thinSOI wafer according to a preferred embodiment of the present invention;

FIG. 3 shows cross section of an SOI bonded wafer;

FIGS. 4A-C show an example of device definition step;

FIGS. 5A-C show an example of defining and sealing the ultra-thinchannels before forming the recessed ESDs;

FIGS. 6A-B show an example of undercutting the ultra-thin layer forforming the recessed ESDs;

FIG. 7 shows an example of filling source/drain voids to form therecessed source/drain extensions;

FIG. 8 shows removal of the remaining protective layer for subsequenttypical semiconductor processing steps.

DETAILED DESCRIPTION

Turning now to the drawings and, more particularly, FIGS. 1A-C, eachshow an example of a field effect transistor (FET) 100, 102, 104 withself aligned recessed source/drain regions and extensions according to apreferred embodiment of the present invention with like elements labeledidentically. In particular, FETs 100, 102, 104 may be typical devices ina circuit, such as a CMOS circuit on an ultra-thin (smaller than orequal to 15 nanometers (10 nm)) semiconductor on insulator (SOI) CMOSchip.

So, in the FET 100 example of FIG. 1A, a gate 106 has spacers 108 ateach side. Preferably, the gate 106 is polysilicon, a metal,silicon-germanium (SiGe), a silicide or combination thereof and thespacers 108 are nitride. The gate 106 is on a gate dielectric layer 110(e.g., oxide) above an ultra-thin channel 112, an ultra-thinsemiconductor layer of strained silicon (SSi), Germanium (Ge), SiGe or,preferably, silicon (Si). The ultra-thin channel 112 is less than 40 nmlong and, preferably, 2-3 times channel thickness or 30 nm. Recessedsource/drain regions 114 are formed on both ends of the ultra-thinchannel 112. The gate 106 has no appreciable direct overlap with therecessed source/drains (ESDs) 114, which have essentially uniformthickness. The source/drain regions 114 extend into the insulator (e.g.,oxide) layer 116 at least 5 nm and, preferably 20-30 nm, below theultra-thin silicon channel 112 for an overall source/drain thickness of50 70 nm. In this embodiment, the extensions inherently form in eitherend of the channel 112, such that the device 100 is self aligned to therecessed source/drain regions 116 and extensions. The extensions operateto minimize short channel effect, so that high resistance lightly dopeddrains (LDD) regions are unnecessary and so, both device resistance andoverlap capacitance minimized.

In the FET 102 example of FIG. 1B, self-aligned extensions 118 areformed at either end of the channel 112′ where the gate 106 overlaps thesource/drain recesses 114, which also has essentially uniform thickness,e.g., 50 70 nm. Again, since the gate 106 overlaps the self alignedextensions 118, series channel resistance is minimized. In the FET 104example of FIG. 1C, self-aligned extensions 119 are also formed ateither end of the channel 112″ where the gate 106 overlaps thesource/drain recesses 114′. However, in this example, but thesource/drain recesses 114′ have a non-uniform, stepped thickness, beingslightly thinner at extensions 119.

FIG. 2 shows in a flow diagram 120 an example of steps for forming selfaligned FETs (e.g., 100) on an ultra-thin SOI wafer with recessed ESDareas 114 according to a preferred embodiment of the present invention.Beginning with a wafer 122, device regions are formed in step 124 toidentify where devices located, e.g., patterning gates 106 at deviceregions and isolating device regions using shallow trench isolation(STI). Trenches extend through the thin silicon surface layer and intothe underlying layers at least to the depth equal to the desiredthickness of the ESDs 114 and preferably to an underlying semiconductorsubstrate. An isolating material, e.g., nitride is deposited to fill thetrenches. The trench material holds the gate and the channel layer inplace during subsequent processing steps. Next, in step 126 source/drainareas are defined for forming the ESDs 114. In step 128 the source/drainareas are undercut, opening orifices that extend into the underlyinginsulation layer 116. In step 130 source/drain areas 114 are filled withsemiconductor material, e.g., silicon, and device fabrication continuesnormally, e.g., source/drain implant and diffusion, metallization andetc.

FIGS. 3-7 show a first preferred embodiment method 140 of forming FETswith self aligned ESD areas on an ultra-thin SOI wafer, e.g., asprovided in step 120 of FIG. 2. So, FIG. 3 shows the cross section ofthe SOI wafer 140 which is a bonded wafer with a semiconductor substrate142, preferably silicon and may be SSi, Ge, SiGe, or strainedsilicon/silicon germanium (SSi/SiGe). An insulator layer 144 (preferablyoxide) separates a sacrificial layer 146 (preferably a 50 nm layer ofundoped silicon), from the semiconductor substrate 142. A thin insulatorlayer 148 (preferably, a 50 nm nitride layer) separates the ultra-thinchannel layer 150 from the sacrificial layer 146.

FIGS. 4A-C show an example of device definition step 124. FIG. 4A showsa plan view with cross sections through BB and CC shown in FIGS. 4B and4C, respectively. So, a gate dielectric or oxide layer 152 is formed onthe ultra-thin channel layer 150. The gate dielectric layer 152 may bean oxide, oxynitride or any suitable hi-K dielectric material layer and,for simplicity of discussion, is referred to herein as gate oxide. Aconductive gate layer of metal, doped polysilicon, a silicide orcombination thereof, is formed on the gate oxide layer 152. Then, thegate layer is patterned to define gates 154 using a suitable definitiontechnique such as are well known in semiconductor manufacturing. Shallowtrenches 156 are formed around the device perimeters, STI therebydefining device regions 158. Preferably, the STI trenches 156 are etcheddown through the ultra-thin channel layer 150, the thin insulating layer148, the sacrificial layer 146 and the insulator layer 144 and etchinginto the silicon substrate layer 142. A thin insulator layer is formedover the gates 154 and filling the shallow trenches 156. The thininsulator layer is anisotropically etched to form spacers 160 alongsidethe gates 154 and with the STI trenches 156 still filled with insulator.

Finally, a dopant, represented by arrows 162, is implanted into thepolysilicon gates 154. The implant dopes portions 164 of the sacrificiallayer 146 at either side of the polysilicon gates 154. Preferably, thedopant 162 may be boron, preferably, implanted at 20 KeV/10¹⁵ or, BF₂implanted at 100 KeV/10 ¹⁵. Then, the wafer is annealed (preferably at900-1000° C. for 5-10 seconds) to activate the boron dopant in thesacrificial layer 146. Optionally, the dopant implant 162 may be donebefore forming thicker spacers 160 if thicker spacers 160 are necessary,e.g., to better the surface layer 150 from the sides when underlyingmaterial is removed in subsequent steps. A two step implant may be usedto recess the source/drain regions 114 and extensions 118 of the FET 104in FIG. 1C. First, prior to forming spacers 160, boron can be implantedat low energy, e.g., 7 KeV/(5*10¹⁴) or, BF₂ can be implanted at 35KeV/(5*10¹⁴). Then, spacers 160 are formed and boron is implanted at ahigher energy, e.g., 20 KeV/10¹⁵ or, BF₂ can be implanted at 100KeV/10¹⁵.

Next, FIGS. 5A-C show an example of the step 126 in FIG. 2 of definingthe source/drain recess areas 164 and, as a result, the ultra-thinchannels. FIG. 5A shows a plan view with cross sections through BB andCC shown in FIGS. 5B and 5C, respectively. First, using a selectiveetch, e.g., reactive-ion-etch (RIE), orifices 165 are opened at bothends of the device areas 158 through to the silicon substrate layer 142.Next, a wet etch is used to remove the oxide layer 144 under thesacrificial layer, which partially forms a void 166 below thesacrificial layer. A thin protective layer 167, preferably oxide, isdirectionally deposited on the upper wafer surface 168 to protect theultra-thin layer 150. Using for example, high density plasma deposition(HDP), a thin oxide can be deposited only on the top of surface, leavingthe bottom of layer 146 free of oxide. Then, the undoped portion of thesacrificial layer is selectively etched away forming a gap 170 betweenthe doped portions 164. The undoped portion is etched using a selectiveetch with a much slower etch rate for boron doped silicon than forundoped material. Removing the undoped portion completes the void 166and defines the channel 172 thereabove with source/drain areas definedby remaining sacrificial portions 164. The channel 172 is embedded inthe ultra-thin channel layer 150 and contained from below by thininsulator layer 148 and from above by gate oxide 152. If the abovedescribed two step dopant was followed to dope the sacrificial layer146, then, the cross-section of the doped portions 164, is substantiallysimilar to the recessed source/drain regions 114 of FIG. 1C.

FIGS. 6A-B show an example of the step 128 in FIG. 2 of undercutting thesource/drain areas for forming recessed ESDs. The void 168 is filledwith a low-k material such as oxide 174 to reduce short channel effects.Then, any excess oxide is etched back, preferably to expose the sides ofsacrificial layer portions 164. Next, arsenic, represented by arrows 176is implanted into the sacrificial layer portions 164 at a dosesufficient that the sacrificial layer portions 164 are converted fromp-type (p+) to n-type (n+). Another thin layer oxide 177 isdirectionally deposited using HDP to protect the polysilicon gate 154and surface layer 150. The wafer is annealed to convert the p-typeportions 164 to n-type, preferably, with a carrier concentration of atleast 10²⁰ per cubic centimeter (cm³). Then, a selective wet etch with afaster etch rate for n-doped material than for undoped material is usedto remove the n-type portions 164, partially forming a source/drainundercut 178 at each end of the channel 172. Optionally, oxide 174 isremoved, slightly, to extend the source/drain voids 178 under eitherside of the gate 154 as in the example of FET 102 in FIG. 1B or FET 104in FIG. 4C. Preferably, this is done by slightly changing the boronimplant (162 in FIG. 4B) tilt angle, angling the boron inward towardsboth sides of the gate. Alternatively, however, oxide 174 may be etchedslightly, although typically, it is harder to control etching,especially under a gate. Preferably, the wet etch also removes bondinglayer 148 above the former location of n-type portions 164 to completethe source/drain undercuts 178. Alternately, however, the exposed (byremoval of the n-type portions 164′) portions of the bonding layer areremoved with a suitable additional wet etch to complete the source/drainundercuts 178.

FIG. 7 shows an example of filling source/drain voids 178 to form therecessed source/drain extensions 180, step 130 in FIG. 2. Preferably,silicon is selectively epitaxially grown to fill the source/drainundercuts 178, thus forming a self aligned device with recessedsource/drain and extension areas 180 at each end of the channel 172. InFIG. 8, after a wet etch is used strip away the remaining protectivelayer 167, completing the device for subsequent typical semiconductorprocessing steps, e.g., implanting n-type and p-type source/draindiffusion, metallization and etc.

Advantageously, a preferred embodiment FET is self aligned with externalseries resistance and parasitic capacitance minimized. Further, thepresent invention has application to ultra-thin SOI, as the ultra-thinsurface silicon layer is thinned to 10 nm and beyond and as devicechannel lengths shrink below 40 nm. Also, the preferred device structureis such that the device operates with lower effective fields, whichallows higher carrier mobility and, correspondingly, higher channelcurrent. In addition, preferred embodiment ultra-thin SOI devices haveimproved short channel effect characteristics including a steepersubthreshold current swing for improved performance. Further, preferredembodiment devices achieve this without the channel resistance andparasitic capacitance penalties incurred with prior art devices.

While the invention has been described in terms of preferredembodiments, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theappended claims.

1. A field effect transistor (FET) comprising: a thin channel having afirst thickness; a gate disposed above said thin channel; a source/drainregion in a recess at each end of said thin channel and substantiallythicker than said thin channel; and a source/drain extension betweensaid thin channel and a corresponding said source/drain region, eachsaid source/drain extension and said corresponding source/drain regionbeing aligned to said gate and said thin channel.
 2. A FET as in claim1, wherein said recess extends under said gate at said each end.
 3. AFET as in claim 1, wherein said recess partially extends under said gateat said each end.
 4. A FET as in claim 1, wherein an upper surface ofeach said source/drain region is substantially coplanar with an uppersurface of said thin channel.
 5. A FET as in claim 1, wherein said thinchannel is a semiconductor material selected from a group consisting ofsilicon (Si), Germanium (Ge), SiGe and strained silicon (SSi).
 6. A FETas in claim 5, wherein said thin channel is strained silicon.
 7. A FETas in claim 5, wherein said gate is made from a material comprisingpolysilicon.
 8. A FET as in claim 5, wherein said gate is made from amaterial comprising a silicide.
 9. A FET as in claim 1, wherein saidthin channel is <15 nm thick.
 10. A FET as in claim 9, wherein said thinchannel is 10 nm thick.
 11. A FET as in claim 9, wherein said thinchannel is <40 nm long.
 12. A FET as in claim 11, wherein said thinchannel is 30 nm long.
 13. A FET as in claim 9, wherein a lower surfaceof said recess is >5 nm below said thin channel.
 14. A FET as in claim13, wherein said lower surface is 40 nm below said thin channel.
 15. AFET as in claim 1, wherein said FET is disposed on an insulating layerand said insulating layer is disposed on a semiconductor substrate. 16.A FET as in claim 15, wherein said semiconductor substrate comprises asilicon substrate.
 17. A FET as in claim 15, wherein said semiconductorsubstrate comprises a strained silicon/silicon germanium (SSi/SiGe)substrate.
 18. An integrated circuit (IC) on a silicon on insulator(SOI) chip, said IC including a plurality of field effect transistors(FETs) disposed on an insulating layer, said insulating layer being on asemiconductor substrate, each of said FETs comprising: a thin channel,said thin channel being a thin semiconductor layer and having a firstthickness; a gate disposed above said thin channel; a source/drainregion in a recess at each end of said thin channel and substantiallythicker than said thin channel; and a source/drain extension betweensaid thin channel and a corresponding said source/drain region, eachsaid source/drain extension and said corresponding source/drain regionbeing aligned to said gate and said thin channel.
 19. An IC as in claim18, wherein each said recess extends under a corresponding said gate atsaid either end.
 20. An IC as in claim 18, wherein each said recesspartially extends under a corresponding said gate at said either end.21. An IC as in claim 18, wherein said thin semiconductor layer is alayer of material selected from a group consisting of silicon (Si),Germanium (Ge), SiGe and strained silicon.
 22. An IC as in claim 21,wherein said thin channel is silicon and said gate is polysilicon. 23.An IC as in claim 22, wherein said thin channel is strained silicon. 24.An IC as in claim 22, wherein said thin channel is <15 nm thick.
 25. AnIC as in claim 24, wherein a lower surface of each said recess is >5 nmbelow said thin channel.
 26. An IC as in claim 25, wherein said thinchannel is <40 nm long.
 27. An IC as in claim 26, wherein saidsemiconductor substrate is a silicon substrate.
 28. An IC as in claim27, wherein said semiconductor substrate is a strained silicon/silicongermanium (SSi/SiGe) substrate.